Biomedical Engineering Reference
In-Depth Information
that lie in a plane. This is a great pity because a noncoplanar app-
roach often has advantages. In our experience with proton beam
therapy at the Harvard Cyclotron Laboratory, we found that we
employed non-coplanar approaches in about two-thirds of the
treatments.
Determination of beam weight(s)
Not all beams need to be equally weighted
−
that is, need to deliver
approximately the same dose to the target volume. Often, one
chooses to weight some beams more heavily than others. Just how
the weighting is decided upon is a matter of experience and trial and
error, involving expert judgment - bolstered sometimes by rules of
thumb developed in the clinic. The optimization techniques described
in Chapter 9 for IMRT can equally be applied to the problem of
choosing beam weights in uniform-intensity radiation therapy
(Niemierko, 1992).
Iterations of the planning process
Rarely does a satisfactory plan, even when developed by a very
experienced planner, emerge after the first attempt. The exception
to this is when standard planning approaches (class-solutions) are
required by protocol. Normally, the planner will work on several
plans, choose the two or three best, and consult with the clinician as
to which he or she prefers.
E
NVIRONMENTALLY
F
RIENDLY
D
OSE
D
ISPOSAL
Let me start at the outset of this section to state what I believe to be
the central tenet of treatment planning. Namely, that
the planner's
job is to decide how to dispose of (i.e., distribute) the dose that must
inevitably be delivered outside the target volume in the best manner
possible
. The terms “dose dumping” and “dose littering” have also
been coined for this phenomenon. Planners are, in the last analysis,
disposal engineers.
immediate conclusion was that noncoplanar beams were probably used at
best infrequently at that facility.
